When did January 1 become the first day of the new year?
The New Year didn’t begin on January 1 until the year 46 BCE, when Julius Caesar added 90 days to the year, creating the Julian calendar. Before that, the year began in late March with the first full moon after the spring equinox. Caesar changed it to honor Janus, the god of beginnings and namesake of the month January. In ancient Rome, New Year’s celebrations consisted of gifts and decorations, as well as sacrifices to Janus.
Source: History.com